S cones are more different from the M and L cones

Although S cone is one of the three types of cones, it is quite different from the L and M cones in several major manners.

• The S cone's sensitivity curve (with peak at 437 mm) is very different from the sensitivity curves of the L and M cones which are quite similar with their peaks very close to each other (at 564 mm and 533 mm respectively) and overlap over a wide range of the visible wavelengths.
• The population of the S cones is much smaller than that of the L or M cones. The S cones consititute only about 10 % of the overall cone population, while the remaining 90 % are L and M cones, with 2 to 4 times more L cones than M cones.
• S cones are totally absent in the foveal area (and so are the rods) where there is a high concentration of M and L cones.
• The genes of the photoreceptors lie on different chromosomes. The gene for rod lies on chromosome 8, the gene for S cone lies on chromosome 7, both among the 22 nonsex chromosomes or autosomes, while the genes for both L and M cones lie on the X chromosome, one of the two sex chromosomes (X and Y). This is why red/green color blindness is much more commen than other color blindness and also why the probability of color blindness is much higher in male (XY) than in female (XX).

Evidences show that the difference between the S cones and the M/L cones is due to evolutionary reasons, as the separation of the S cones and the M/L cones probably occured much earlier than the separation of the M and L cones (for details see Rodieck 1998).